1. Field of the Invention
The invention relates to a photovoltaic element in which a plurality of photovoltaic cells are enclosed between two plastic plates.
2. Discussion of the Background
Photovoltaic elements containing photovoltaic cells embedded in plastic between transparent panes are generally known. These elements are used for applications such as roofing and illuminating bus stops.
German Patent Application 4443317 describes a photovoltaic element in which a plurality of photovoltaic cells are embedded in an elastic plastic material between two polymethyl methacrylate plates. The photovoltaic cells are protected against ingress of water and against mechanical influences, such as impacts.
German Patent Application 3801989 describes a cladding panel of insulating glass in which photovoltaic solar generators in the air space between two glass panes are attached to one of the panes using an elastic plastic. Thereafter the solar generator is encapsulated with hot-melt adhesive or enveloped with sheeting.
Adhesives for methyl methacrylate plastics are known. The adhesive bonding of polymethyl methacrylate parts by means of nitroethane or nitromethane is known from German Unexamined Application (DE-OS) 2737270. Polymerization adhesives that are based on polymethyl methacrylate or methyl methacrylate and that can be cured by means of UV or visible light are described in, for example, European Patent Applications 436205 or 548740. The use of lower alkyl formates in volatile adhesives or adhesive lacquers based on polymethyl methacrylate is known from European Patent Application 663419. An advantageous adhesive bonding technique for polymethyl methacrylate parts using solvent adhesives in combination with viscous polymerization adhesives is described in European Patent Application 675184.
Photovoltaic elements as described in German Patent Application 3801989 or 4443317 offer good protection to the photovoltaic cells, since these are fixed between the plates by means of an embedding agent. This fixation method is expensive, however. Above and beyond this, especially under conditions of prolonged exposure, the danger exists that the embedding medium will interact chemically with the embedded cells, possibly destroying the cells. Furthermore, impact loads are transmitted from the surrounding plates via the embedding medium to the photovoltaic cells, possibly causing damage. A further problem is presented by moisture condensation in the cavities, possibly leading to fogging of the panes and thus to a reduction of photovoltaic output and even to destruction of the cells.
Therefore, a need exists to provide an improved photovoltaic element, which is simple to manufacture and protects the cells effectively against chemical or mechanical influences.
It is an object of the present invention to provide a photovoltaic element that protects the photovoltaic cells against chemical and mechanical influences and is easy to manufacture.
This and other objects have been achieved according to the present invention, the first embodiment of which provides a photovoltaic element, comprising:
two plane or curved plates (1) and (2), which are joined by a circumferential seal (3) at a spacing ranging from 2 to 20 mm, thus forming a cavity (4) in which there are enclosed a plurality of photovoltaic cells (5), each of which is attached to only one of the two plates (2) using an adhesive (6), at least this plate (2) being transparent;
wherein both plates (1) and (2) are made of plastic that is permeable to water vapor and the adhesive with which the photovoltaic cells are attached covers at most 30% of the area of the individual photovoltaic cells.